Читать книгу Cucurbits - James R. Myers - Страница 14

Cucurbit leaves are usually simple (i.e. not divided into leaflets), palmately veined and shallowly to deeply three- to seven-lobed. There is usually one leaf per stem node. Along the stem, leaves are helically arranged with a phyllotaxy of 2/5; in other words, there are two twists of the stem, which segment contains five leaves, before one leaf is directly above another. This means that the angle of divergence between neighbouring leaves is 144° (2/5 of 360°).

Leaf stomata are mostly anomocytic, lacking subsidiary cells. The petiole in cross-section often has a crescent or ring of unequal vascular bundles, the larger ones bicollateral. Stipules at the base of the petiole are typically absent, but have been transformed into photosynthetic thorns in Acanthosicyos. Extrafloral nectaries, which frequently attract ants, occur on some cucurbit leaves (e.g. ivy gourd).

Succulent leaves are rare, even among the xerophytic cucurbits. Those of Xerosicyos have large water-storage cells in the inner mesophyll and perform crassulacean acid metabolism (CAM). However, the deciduous leaves of Seyrigia only perform C₃ photosynthesis even though the succulent stem performs CAM. The large ephemeral leaves of most cucurbits adapted to an arid environment avoid heat damage by maintaining high levels of transpirational cooling (Rundel and Franklin, 1991).